A snake is someone who you think is sincere and really nice, but then turns out to be a backstabber.
Someone who acts like your best friend, but who actually is the opposite.

"We all know somebody who is a doubletalker, a gossip and a generally untrustworthy person. They are cruel to their enemies and even meaner to their friends - only with the friends the mean streak only comes out when their backs are turned. This type of friend is all sweetness and sugar to people's faces but act ugly and spiteful behind their backs. We call people like this "backstabbers" because they sneak around portraying themselves as something they are not, a friend." - www.teenadvice.about.com